I have a .torrent file that's 553KB in size that I'm trying to upload. It's my first torrent so I just now realized that the max size is about 270KB. However, I've seen files even bigger than 1TB uploaded on TPB so what gives? How do they reduce their .torrent size so drastically?

Also, is there a way to calculate how large a .torrent file will end up if you know how large the files you're putting in are? For example, I have 53.2GB of files, can I calculate the file size of the .torrent created from those files?

No, you cannot calculate how big a torrent file will be simply based on the size of the content.

A torrent is made up of meta-data, so its size is determined by many things.

For that amount of data, the torrent file itself will largely be filled with piece hashes, so the most effective way to slim it down will be to increase the piece size, so the total number of pieces will go down.

If that doesn't work, you'll have to split the content up into multiple torrents.